More Studies

1. Naps as integral parts of the wake time within the human sleep-wake cycle.

   ASCHOFF J.
   J Biol Rhythms 1994;9(2):145-55.
   Max-Planck-Institut fur Verhaltensphysiologie, Andechs, Germany.

   Protocol: Thirteen subjects lived singly in an isolation unit without temporal cues for an average time span of 32 days. They signaled the times when they woke up, took a meal, and retired, noting in their diaries what kind of sleep (nap or main sleep) they were going to have.

   Results: In five subjects the free-running circadian rhythms remained internally synchronized. The other eight subjects became internally desynchronized, with different periods in the sleep-wake cycle and in the rhythm of body temperature, or developed a “circa-bi-dian” rhythmicity (i.e., a state of the circadian system in which the temperature rhythm regains synchrony with the sleep-wake cycle in a 2:1 ratio). The duration of naps was positively correlated with the duration of wake time (after subtraction of the nap), as well as with the duration of wakefulness preceding the nap. In contrast, the duration of main sleep was negatively correlated with the preceding wake time.

   Conclusions: It is concluded that naps are integral parts of the wake time, and that they follow the rules known from intermeal intervals and the perception of long time intervals such as 1 hr.

2. The effects of a 20 min nap in the mid-afternoon on mood, performance and EEG activity.

   HAYASHI M, WATANABE M, HORI T.
   Clin Neurophysiol 1999;110(2):272-9.
   Department of Behavioral Sciences, Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-hiroshima, Japan

   Objective: The aim of the study is to examine the effects of a 20 min nap in the mid-afternoon on mood, performance and EEG activities.

   Methods: Seven young adults who had normal sleep-wake habits without habitual daytime napping participated in the study. They underwent Nap and No-nap conditions at intervals of 1 week. After a nocturnal sleep recording (00:00-08:00 h), their EEG recordings during relaxed wakefulness, and their mood, performance and self-ratings of performance level were measured every 20 min from 10:00 to 18:00 h. For the nap condition, they went to bed at 14:00 h and were awakened when 20 min had elapsed from the onset of sleep stage 1. For the No-nap condition, they took a rest without sleep by sitting on a semi-reclining chair.

   Results: All of the subjects were awakened from sleep stage 2 during the nap. The 20 min nap improved the subjective sleepiness, performance level and self-confidence of their task performance. The nap also suppressed EEG alpha activity during eyes-open wakefulness.

   Conclusions: The results suggest that a short 20 min nap in the mid-afternoon had positive effects upon the maintenance of the daytime vigilance level.

3. The effects of a short daytime nap after restricted night sleep.

   GILLBERG M, KECKLUND G, AXELSSON J, AKERSTEDT T.
   Sleep 1996;19(7):570-5.
   Karolinska Institute, Stockholm, Sweden

   Study: Eight subjects participated on three occasions in a study investigating the effect of a 30 minute daytime nap opportunity on alertness/sleepiness.

   Protocols: The baseline condition was a normal home sleep (7.5 hours, with bedtime at 2300 hours). Sleep during the other two conditions was between 2400 hours and 0400 hours. During one of the two 4-hour conditions, a short nap was allowed (between 1045 hours and 1115 hours). Self-ratings of sleepiness/alertness (Karolinska Sleepiness Scale) were recorded every hour. At 10, 12, and 15 hours, the subjects performed a 28-minute visual vigilance task. Electroencephalograms (EEG) and electrooculograms (EOG) were recorded continuously, including during a 10-minute standardized recording procedure at the beginning of each day. Mean total sleep time during the nap was 19.8 (standard error 2.4) minutes.

   Results: Compared to baseline, EEG/EOG sleepiness and subjective sleepiness were significantly higher and vigilance performance at 10 hours lower, respectively, after the two short sleeps.

   Conclusions: The nap brought performance to baseline levels, and subjective sleepiness decreased significantly. It was concluded that the short nap had a clear positive effect on alertness.

4. The role of a short post-lunch nap in improving cognitive, motor, and sprint

   WATERHOUSE J, ATKINSON G, EDWARDS B, REILLY T.
   J Sports Sci 2007;25(14):1557-66.
   Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, Liverpool, UK. waterhouseathome@hotmail.com

   Overview: The aim of this study was to determine the effects of a post-lunch nap on subjective alertness and performance following partial sleep loss.

   Protocol: Ten healthy males (mean age 23.3 years, s = 3.4) either napped or sat quietly from 13:00 to 13:30 h after a night of shortened sleep (sleep 23:00-03:00 h only). Thirty minutes after the afternoon nap or control (no-nap) condition, alertness, short-term memory, intra-aural temperature, heart rate, choice reaction time, grip strength, and times for 2-m and 20-m sprints were recorded.

   Results:The afternoon nap lowered heart rate and intra-aural temperature. Alertness, sleepiness, short-term memory, and accuracy at the 8-choice reaction time test were improved by napping (P < 0.05), but mean reaction times and grip strength were not affected (P > 0.05). Sprint times were improved. Mean time for the 2-m sprints fell from 1.060 s (s(x) = 0.018) to 1.019 s (s(x) = 0.019) (P = 0.031 paired t-test); mean time for the 20-m sprints fell from 3.971 s (s(x) = 0.054) to 3.878 s (s(x) = 0.047) (P = 0.013). These results indicate that a post-lunch nap improves alertness and aspects of mental and physical performance following partial sleep loss, and have implications for athletes with restricted sleep during training or before competition.

5. The use of prophylactic naps and caffeine to maintain performance during a continuous operation.

   BONNET MH, ARAND DL.
   Ergonomics 1994;37(6):1009-20.
   Dayton VA Hospital, Wright State University, OH 45428.

   Protocol: After a normal baseline night of sleep and a morning of baseline test performance, 24 young adult male subjects returned to bed from 16:00-20:00 prior to a 24 h period of sleep loss. Twelve subjects received caffeine 200 mg at 01:30 and 07:30.

   Results: Performance tests (correctly completed addition problems, vigilance sensitivity, and logical reasoning correct responses) all indicated maintenance of baseline performance levels in the caffeine group after administration of caffeine while performance declined in the placebo group. Similar results were found for the Multiple Sleep Latency Test and Oral Temperature, which both remained near baseline levels throughout the observation period in subjects receiving caffeine.

   Conclusions: The results indicated that the combination of a prophylactic nap and caffeine was more effective in maintaining nocturnal alertness and performance than was the nap alone. Of more interest was the fact that the group which was given the combination of nap and caffeine was able to maintain alertness and performance at very close to baseline levels throughout a 24 h period without sleep.

6. Short nap versus short rest: recuperative effects during VDT work.

   HAYASHI M, CHIKAZAWA Y, HORI T.
   Ergonomics 2004;47(14):1549-60. Department of Behavioral Sciences, Faculty of Integrated Arts and Sciences, Hiroshima University, Higashi-hiroshima, Japan. mhayasi@hiroshima-u.ac.jp

   Study: The effects of a 20-min nap during 2 h of visual display terminal (VDT) work were examined.

   Methods: Ten young healthy adults took a 20-min nap or a 20-min rest I h after VDT work, followed by another 1 h of VDT work.

   Results: A 20-min rest temporarily restored subjective sleepiness, but it deteriorated during the additional 1 h of work. In contrast, a 20-min nap maintained subjective alertness and performance level at a higher level and mental fatigue at a lower level for the additional 1 h of work.

   Conclusions: These results suggest that a short nap would be useful to both fatigue recovery and fatigue prevention during continuous VDT work. The present findings may provide a new work/rest strategy.

7. Habitual napping moderates motor performance improvements following a short daytime nap.

   MILNER CE, FOGEL SM, COTE KA.
   Biol Psychol 2006 Mar 13
   Brock University, Sleep Research Laboratory, St. Catharines, Ont., Canada

   Study:The effect of napping on motor performance was examined in habitual and non-habitual nappers who were randomly assigned to a nap or reading condition. Motor procedural learning and auditory discrimination tasks were administered pre- and post-condition.

   Results: Both groups reported improved alertness post-nap, but not post-reading. Non-habitual nappers fell asleep faster and tended to have greater sleep efficiency, but did not differ from habitual nappers on other sleep architecture variables. Habitual nappers had greater alpha and theta EEG power in stage 1, and greater delta, alpha and sigma power in stage 2 sleep. Motor performance deteriorated for non-habitual nappers who napped, but improved for all others. The number of sleep spindles and sigma power (13.5-15Hz) significantly predicted motor performance following the nap, for habitual nappers only.

   Conclusions: Results indicate that motor learning was consolidated in a brief nap and was associated with stage 2 spindles, but only for those who habitually take naps.

8. Siesta in healthy adults and coronary mortality in the general population.

   NASKA A, OIKONOMOU E, TRICHOPOULOU A, PSALTOPOULOU T, TRICHOPOULOS D.
   Arch Intern Med 2007;167(3):296-301.
   Department of Hygiene and Epidemiology, University of Athens Medical School, Athens, Greece

   Background: Midday napping (siesta) is common in populations with low coronary mortality, but epidemiological studies have generated conflicting results. We have undertaken an analysis based on a sizable cohort with a high frequency of napping and information on potentially confounding variables including reported comorbidity, physical activity, and diet.

   Methods: Among participants in a general population cohort (the Greek European Prospective Investigation into Cancer and Nutrition [EPIC] cohort), 23,681 individuals who at enrollment had no history of coronary heart disease, stroke, or cancer and had complete information on frequency and duration of midday napping, as well as on potentially confounding variables, were followed up for a mean of 6.32 years. Data were modeled through Cox regression, using time to coronary death and treating deaths from other causes as censoring events as outcomes.

   Results: Among men and women, when controlling for potential confounders and using those not taking siesta as a referent category, those taking a siesta of any frequency or duration had a coronary mortality ratio (MR) of 0.66 (95% confidence interval [CI], 0.45-0.97). Specifically, those occasionally napping had a 12% lower coronary mortality (MR, 0.88; 95% CI, 0.48-1.60), whereas those systematically napping had a 37% lower coronary mortality (MR, 0.63; 95% CI, 0.42-0.93). Among men, the inverse association was stronger when the analysis was restricted to those who were currently working at enrollment, whereas among women, a similar analysis was not possible because of the small number of deaths.

   Conclusions: After controlling for potential confounders, siesta in apparently healthy individuals is inversely associated with coronary mortality, and the association was particularly evident among working men

9. Sleep-Dependent Learning: a Nap is as Good as a Night

   Mednick, S., Nakayama, K., Stickgold, R., Nature Neuroscience
   , published online June 2003

   “We now report that sleep-dependent learning of a texture discrimination task can be accomplished in humans by brief (60-90 min) naps containing both slow-wave sleep (SWS) and rapid eye movement (REM) sleep.”

   Read the full study results.